This project investigates primate biobehavioral development through comparative longitudinal studies of rhesus and capuchin monkeys, with special emphasis on characterizing individual patterns of differential behavioral and physiological responses to environmental novelty and challenge and on determining long-term developmental consequences for individuals of different genetic backgrounds reared in different physical and social environments. Research carried out during FY99 (a) documented significant differences between rhesus monkeys of Indian vs. Chinese ancestry in the relative frequency of specific polymorphisms of the serotonin transporter gene (5-HTT); (b) demonstrated significant interactions between 5-HTT polymorphisms and early rearing history for visual orienting capabilities and state control in rhesus monkey neonates and infants and for cerebrospinal fluid (CSF) concentrations of the primary central serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) and aggression in rhesus monkey juveniles and adults; (c) characterized disruption of normal circadian activity patterns in free- ranging rhesus monkey adult males with low CSF 5-HIAA concentrations, replicating previous findings from captive subjects, and documented higher mortality rates for those males than for other males in their birth cohort; (d) identified significant species differences between rhesus monkeys and pigtail macaques in CSF 5-HIAA concentrations that were predictive of differences between these two macaque species in overall aggressivity and general sociality; (e) demonstrated that salivary prolactin concentrations in rhesus monkey adults are highly correlated with CSF 5-HIAA concentrations, providing a possible noninvasive procedure for assessing serotonergic function; (f) documented that left- handedness in rhesus monkeys is associated with excessive hypothalamic-pituitary-adrenal (HPA) activity, fearfulness, and impaired immune function (as assessed by CD4% and CD4/CD8 ratios); (g) demonstrated that the CRH type 1 receptor antagonist analarmin suppresses HPA activity and behavioral manifestations of fearfulness in adult male rhesus monkeys; (h) characterized developmentally stable individual differences in HPA responsiveness among capuchin monkey infants and juveniles, despite significant developmental changes in their HPA activity; and (i) identified significant gender differences among capuchin monkey juveniles and young adults in play behavior, social vigilance, and in relative amount of time spent interacting with mothers and peers that parallel gender differences previously reported for rhesus monkeys, despite major differences between the two species in overall rates of social activity and in social group structure and dynamics. - primates, biobehavioral development, individual differences, gene-environment interactions, serotonin, HPA activity